1. Field of the Invention
This invention relates to a furnace system employing a flue damper which closes when the furnace is off to conserve heat in the combustion chamber and a safety circuit for controlling the damper including a gas flow detector switch of novel construction.
2. Prior Art
Dampers have previously been employed in the exhaust flues of central heating furnaces and the like, which cycle on and off, to close the flues when the furnaces are off. This conserves fuel by retaining heated air in the furnace bonnet to increase the time required for the bonnet temperature to reach atmospheric temperature, and by preventing the flow of hot air from the interior of the building being heated, adjacent the furnace, through the furnace and flue to the atmosphere. Tests have shown that these valves will reduce fuel consumption by as much as about a quarter while improving the comfort of the building being heated by eliminating drafts caused by air flow through the furnace and decreasing the rate of furnace cycling. The savings in fuel cost repay the relatively modest costs of these valves in a very short period of time so that the purchase of the valves as a part of a new furnace system or as an add-on to an existing system is an excellent investment.
Despite these advantages flue damper valves have not achieved any widespread use, primarily because of the hazardous condition which occurs if the valve malfunctions and remains closed while the furnace is fired. The noxious and oxygen deficient exhaust gases then back up in the flue and begin to overflow into the building, creating the danger of asphyxiating the occupants. Most proposed flue damper systems have provided an electric actuator for the damper controlled by the same signal that energizes the electric fuel valve. These systems are susceptible to the actuator failing to move the valve to an opened position when the control signal is received or by the gas shutoff valve sticking in an open position after its control signal is terminated. The possibility of the valve failing to open can be substantially minimized by biasing the valve into an open position with a gravity biased mechanism, requiring the presence of an electrical signal to move the valve to the closed position. The problem of the gas valve sticking in an open position is more difficult to solve and is a more common occurrence. It has been proposed to provide a pair of valves in the gas line so that either one can close off the flow but such systems provide no indication of the malfunction of one of the valves which leaves the system open to dangerous operation upon failure of the other valve. U.S. Pat. No. 2,537,082 proposes a system wherein the flue valve and the gas flow valve are both controlled with a common mechanical actuator. While this system might be utilized with new furnaces specifically designed for its incorporation, it can seldom be installed in an existing furnace without extensive reworking of the gas line and the flue.